U.S. Pat. No. 6,869,474 to Perez-Pena et al, incorporated herein by reference, discusses extremely fast setting of cementitious compositions for producing cement-based products such as cement boards achieved by adding an alkanolamine to hydraulic cement such as portland cement, and forming a slurry with water under conditions that provide an initial slurry temperature of at least 90° F. (32° C.). Additional reactive materials may be included such as high alumina cement, calcium sulfate and a pozzolanic material such as fly ash. The extremely rapid set permits rapid production of cementitious products. Triethanolamine additions have been found to be a very powerful accelerator capable of producing formulations with relatively short final setting times with increased levels of fly ash and gypsum and without the need of calcium aluminate cements. However, formulations with triethanolamine contain mainly hydraulic cements such as portland cement and gypsum as the reactive powder, which limits the availability of aluminate phases compared to the aluminate phases produced by the activation of fly ash materials in the present invention.
Pending U.S. patent application Ser. No. 11/758,947 filed Jun. 6, 2007 of Perez-Pena et al, incorporated herein by reference, discusses extremely fast setting of cementitious compositions with early-age compressive strength for producing cement-based products such as cement boards achieved by adding an alkanolamine and a phosphate to a hydraulic cement such as portland cement, and forming a slurry with water under conditions that provide an initial slurry temperature of at least 90° F. (32° C.). Additional reactive materials may be included such as high alumina cement, calcium sulfate and a pozzolanic material such as fly ash. Again, all of the compositions contained a significant amount of hydraulic cement and gypsum.
Pending U.S. patent application Ser. No. 12/237,634 filed Sep. 25, 2008 of Perez-Pena discloses formulations using fly ash and alkali metal salts of citric acid such as sodium citrate to form concrete mixes with fast setting time and relatively high early age compressive strength. One of the challenges encountered with the activated fly ash binders described in this application, is an apparent pessimum interaction between these binders and the traditional foaming systems used to entrain air and thereby make lightweight boards. The fly ash based binders which have been made with traditional foams in accordance with this disclosed method have suffered foam collapsing and/or drastic strength reduction.
U.S. Pat. No. 4,488,909 to Galer et al, incorporated herein by reference, discusses cementitious compositions capable of rapid setting. The compositions permit high speed production of carbon dioxide resistant products by forming essentially all of the potential ettringite within about 20 minutes after the composition is mixed with water. The essential components of the cementitious composition are portland cement, high alumina cement, calcium sulfate and lime. Pozzolans such as fly ash, montmorillonite clay, diatomaceous earth and pumicite may be added up to about 25%. The cement composition includes about 14 to 21 wt % high alumina cement, which in combination with the other components makes possible the early formation of ettringite and other calcium aluminate hydrates responsible for early setting of the cementitious mixture. In their invention, Galer et al provided aluminates using high alumina cement (HAC) and sulfate ions using gypsum to form ettringite and achieve rapid setting of their cementitious mixture.
Ettringite is a calcium aluminum sulfate compound having the formula Ca6Al2(SO4)3.32H2O or alternatively 3 CaO.Al2O3.3CaSO4.32H2O. Ettringite forms as long needle-like crystals and provides rapid early strength to cement boards, so that they can be handled soon after being poured into a mold or over a continuous casting and forming belt.
In general, Galer et al's rapid setting formulation suffers from several limitations. These limitations, as highlighted below, are even more of a concern for the production of low cost concrete products such as lightweight aggregates because it uses relatively expensive high alumina cements to provide aluminate phases.
U.S. Pat. No. 5,536,310 to Brook et al disclose a cementitious composition containing 10-30 parts by weight (pbw) of a hydraulic cement such as portland cement, 50-80 pbw fly ash, and 0.5-8.0 pbw expressed as a free acid of a carboxylic acid such as citric acid or alkali metal salts thereof, e.g., tripotassium citrate or trisodium citrate, with other conventional additives, including retarder additives such as boric acid or borax, which are used to accelerate the reaction and setting time of the composition to overcome the disclosed disadvantageous of using a high fly ash content in cement compositions.
U.S. Pat. No. 5,536,458 to Brook et al disclose a cementitious composition containing a hydraulic cement such as portland cement, 70-80 parts by weight fly ash, and 0.5-8.0 pbw of a free carboxylic acid such as citric acid or an alkali metal salts thereof e.g. potassium citrate or sodium citrate, with other conventional additives including retarder additives such as boric acid or borax, which are used to accelerate the reaction and setting time of the composition to overcome the known disadvantageous of using a high fly ash content in cement compositions.
U.S. Pat. No. 4,494,990 to Harris discloses a cementitious mixture of portland cement e.g. 25-60 pbw, fly ash e.g. 3-50 pbw and less than 1 pbw of sodium citrate.
U.S. Pat. No. 6,827,776 to Boggs et al disclose a hydraulic cement composition comprising portland cement, fly ash, which has a setting time controlled by pH of an activator slurry of an acid, preferably citric acid, and a base which can be an alkali or alkaline earth metal hydroxide or salt of the acid component.
U.S. Pat. No. 5,490,889 to Kirkpatrick et al disclose a blended hydraulic cement consisting of water, fly ash (50.33-83.63 pbw), portland cement, ground silica, boric acid, borax, citric acid (0.04-2.85 pbw) and an alkali metal activator, e.g. lithium hydroxide (LiOH) or potassium hydroxide.
U.S. Pat. No. 5,997,632 to Styron discloses a hydraulic cement composition containing 88-98 wt. % fly ash, 1-10 wt. % portland cement and from about 0.1-4.0 wt. % citric acid. Lime to achieve a desirable minimum lime content of 21% is provided by the subbituminuous fly ash or the sub-bituminous fly ash in combination with a beneficiating agent. In addition to citric acid Styron uses an alkali source such as potassium or sodium hydroxide.
The final setting times of the cementitious mixtures of prior art products are typically greater than 9 minutes and can extend to 2-3 hours for standard concrete products. The final setting time is normally defined as the time in which the cementitious mixtures set to the extent that the concrete products made thereof can be handled and stacked, although chemical reactions may continue for extended periods.
There is a need to find a method to reduce the weight of fly ash based binder mixes so these formulations can be used to manufacture of lightweight cementitious concrete products for applications such as backer board and other wall or ceiling applications with improved strength. The present method has developed formulations with enhanced compressive strength at reduced weight and with reduced cost.
When lightweight concrete is made, a lightweight aggregate like expanded clay or perlite is generally used rather than sandy gravel or crushed stone. The expanded clay/perlie particles (nodules) are produced by a sophisticated pyrogenic process whereby geochemically specific clay or perlite is expanded in a rotary kiln at high temperatures. The expanded clay or perlite particles are extremely lightweight granular aggregate with a hard vitrified outer shell and an air filled honeycombed inner core. The expansion of the combined water in the crude clay or perlite (rock), results from the rapid heating of the crude rock to temperatures above 1600° F. (871° C.), when the rock cracks and combined water vaporizes like popped corn.
The present invention provides a method of making fast setting cementitious slurry used to manufacture lightweight aggregate particles (nodules) at a relatively low temperature. The lightweight aggregate particles (nodules) can form in situ in a cementitious mixture. Setting the mixture results in a solid product comprising particles (nodules) formed in situ in the matrix of cementitious material. This product can be used as is or crushed to form loose lightweight aggregate particles.
The particles are extremely lightweight with a hard outer cementitious shell with an air filled crystalline inner core. The lightweight particles provide a low energy, low cost lightweight filler which is ideal for manufacturing lightweight cement panels, block or other lightweight concrete articles.
The invention also provides a lightweight cementitious composition with reduced weight and enhanced early and final compressive strength. The cementitious composition is formed from a foamed binder solution containing sodium citrates, sodium silicates, foaming agents, foam stabilizer and a reactive powder comprising Class C fly ash and calcium sulfate.
The present invention includes a method of providing a lightweight cementitious mixture having rapid set, improved compressive strength and water durability comprising: mixing at ambient or above ambient temperatures, water, cementitious reactive powder, a set accelerating amount of alkali metal salt of citric acid, and in situ forming a reactive powder lightweight aggregate, wherein the weight ratio of water to reactive powder solids is about 0.17 to 0.35:1.0, or about 0.17 to 0.27:1.0, and more preferably about 0.20 to 0.25:1.0. The cementitious reactive powder comprises fly ash. and a calcium sulfate selected from the group consisting of calcium sulfate hemihydrate, calcium sulfate dihydrate, and mixtures thereof, and preferably no hydraulic cement, e.g., no portland cement. Typically essentially 100 wt. of the fly ash is in the form of class C fly ash and blends of class C and class F fly ash. For purposes of the present specification cements are characterized as hydraulic or non-hydraulic. Hydraulic cements (e.g., Portland cement) harden because of hydration, chemical reactions that occur independently of the mixture's water content; they can harden even underwater or when constantly exposed to wet weather. The chemical reaction that results when the anhydrous cement powder is mixed with water produces hydrates that are not water-soluble. Non-hydraulic cements (e.g., lime, stucco, gypsum/landplaster and gypsum plaster) must be kept dry to retain their strength. Typically the mixture has a wet (water included) density of about 40 to 65 pounds per cubic foot, for example, 46 to 51 pounds per cubic foot.
The method generally further includes setting the mixture to form a solid product containing the in situ formed aggregate particles. The solid product can be used as is or can be broken up, for example by crushing, to form loose aggregate particles of the present invention.
This cementitious reactive powder includes at least fly ash and stucco or gypsum/landplaster (stucco is calcium sulfate hemihydrate, gypsum is calcium sulfate dihydrate) and may also contain ordinary portland cement (OPC), calcium aluminate cement (CAC) (also commonly referred to as aluminous cement or high alumina cement), and a non-fly ash mineral additive. However, typically there is no added ordinary portland cement (OPC) or calcium aluminate cement (CAC).
Class C fly ash generally contains lime. Thus, the reactive powder blend of the cementitious composition is typically free of externally added lime.
The preferred initial slurry temperatures are from room temperature to about 100° F.-115° F. (24° C. to about 38°-46° C.).
The final setting time (i.e., the time after which cementitious boards can be handled) of the cementitious composition as measured according to the Gilmore needle should be at most 20 minutes, preferably 10 to 13 minutes or less, more preferably about 4 to 6 minutes after being mixed with a suitable amount of water. A shorter setting time and higher early age compressive strength helps to increase the production output and lower the product manufacturing cost.
The very fast setting cementitious compositions of this invention can be used for a variety of applications in which rapid hardening and attainment of early strength is desirable. Using the alkali metal salt of citric acid, such as potassium citrate and/or sodium citrate, to accelerate setting of the cementitious composition, when the slurry is formed at elevated temperatures, makes possible increased rate of production of cementitious products such as cement boards.
The dosage of alkali metal citrate in the slurry is in the range of about 1.5 to 6 wt. %, preferably about 1.5 to 4.0 wt. %, more preferably about 2 to 3.5 wt. %, and most preferably about 3.5 wt. % based on the cementitious reactive components of the invention. Sodium citrates are preferred, although potassium citrate or a blend of sodium and potassium citrate can be used. As mentioned above, these weight percents are based on 100 parts by weight of the reactive components (cementitious reactive powder). Thus for example, for 100 pounds of cementitious reactive powder, there may be about 1.5 to 4.0 total pounds of sodium citrates.
A typical cementitious reactive powder of this invention comprises 75 to 100 wt % fly ash and 0 wt. % hydraulic cement. Typically at least half of the fly ash is Type C fly ash.
A cementitious reactive powder of this invention can also comprise class F fly ash up to 46 wt % when mixed with a sufficient amount of class C fly ash, sodium citrate and optional Portland cement of 0 to 20 wt % to make up for the lower compressive strength of the class F fly ash compared to the higher alumina and lime content of the preferred class C fly ash. When higher amounts of class F fly ash are used, i.e. up to 60 wt % of the reactive powder, it has been found the lower compressive strength of the class F fly ash can not be sufficiently increased by only adding class C fly ash to the class F fly ash. Thus, although not preferred, class F fly ash can be used in amounts up to about 60 wt % if Portland Cement, e.g. Type III Portland cement, is used with the class F fly ash to increase the compressive strength by addition of alumina and lime beyond the levels typical found in Class C fly ash. Thus, for example, 46 to 60 wt % class F fly ash could be used with 10 to 32 wt % class C fly ash without compromising compressive strength if about 10 to 29 wt % of additional Type III Portland cement is also used with the fly ash mixture and 2-4 wt. % sodium citrate.
There is a synergistic interaction between the alkali metal citrate and the fly ash. In particular, adding alkali metal citrates to fly ash improves mix fluidity unlike other accelerators such as aluminum sulfate, which can lead to premature stiffening of concrete mixtures.
Other additives, e.g., inert aggregate, may also be present, which are not considered cementitious reactive powder, but are part of the overall cementitious composition. Such other additives include one or more of sand, aggregate, lightweight fillers, water reducing agents such as superplasticizers, set accelerating agents, set retarding agents, air-entraining agents, foaming agents, shrinkage control agents, slurry viscosity modifying agents (thickeners), coloring agents and internal curing agents, may be included as desired depending upon the process ability and application of the cementitious composition of the invention.
The lightweight cementitious compositions of the present invention can be used to make precast concrete building products such as cementitious boards with excellent moisture durability for use in wet and dry locations in buildings. The precast concrete products such as cement boards are made under conditions which provide a rapid setting of the cementitious mixture so that the boards can be handled soon after the cementitious mixture is poured into a stationary or moving form or over a continuously moving belt.
The lightweight cementitious compositions can be used in any concrete product application including concrete panels, flooring, overlays, finishes, capping, as well as patching mixes for concrete roads. The concrete products made with the lightweight compositions of this invention have particular advantages for use which require water durability compared to compositions which contain gypsum and applications which require higher compressive strength than cement containing compositions which have a higher carbon foot print.
Fly ash material is mostly aluminosilicates. Thus, it is theorized the lightweight aggregate of the invention may be similar to that of the most expensive perlite or expanded clay aggregate.
All percentages, ratios and proportions herein are by weight, unless otherwise specified.